Localized and self-aligned punch through stopper doping for finfet

ABSTRACT

A method for doping punch through stoppers (PTSs) includes forming fins in a monocrystalline substrate, forming a dielectric layer at a base portion between the fins and forming spacers on sidewalls of the fins down to a top portion of the dielectric layer. The dielectric layer is recessed to form gaps between the top portion of the dielectric layer and the spacer to expose the fins in the gaps. The fins are doped through the gaps to form PTSs in the fins.

BACKGROUND

Technical Field

The present invention relates to semiconductor devices and processing,and more particularly to devices and methods for doping a self-alignedpunch through stopper to improve device channel performance.

Description of the Related Art

High Ge concentration SiGe is a promising channel material forperformance improvements in p-type field effect transistors (PFETs) dueto the expectation of high mobility. One problem in SiGe PFETfabrication is controlling punch through stopper (PTS) doping. PTSdoping should include n-type dopants (such as, As, P, etc.). N-typedopants are known to have extremely high diffusivity in SiGe. This makesit challenging to control PTS doping below a channel region in bulk SiGefin field effect transistors (finFETs).

SUMMARY

A method for doping punch through stoppers (PTSs) includes forming finsin a monocrystalline substrate, forming a dielectric layer at a baseportion between the fins and forming spacers on sidewalls of the finsdown to a top portion of the dielectric layer. The dielectric layer isrecessed to form gaps between the top portion of the dielectric layerand the spacer to expose the fins in the gaps. The fins are dopedthrough the gaps to form PTSs in the fins.

Another method for doping punch through stoppers (PTSs) includes formingfins in a monocrystalline Si substrate; forming a dielectric layer at abase portion between the fins; mixing Ge in the fins to form SiGe fins,wherein the SiGe fins extend into the monocrystalline Si substrate belowthe dielectric layer; forming spacers on sidewalls of the SiGe fins downto a top portion of the dielectric layer; recessing the dielectric layerto form self-aligned gaps between the top portion of the dielectriclayer and the spacer to expose the SiGe fins in the gaps; plasma dopingthe fins through the gaps to form PTSs in the SiGe fins; activating thePTSs by annealing; filling the gaps with a same dielectric material asthe dielectric layer and etching back the dielectric layer to a positionon the spacers.

A fin device having a punch through stopper (PTS) includes a findisposed between source and drain regions and adjacent to a gatestructure. The fin includes a SiGe fin formed in a monocrystalline Sisubstrate. The SiGe fin has a portion extending above a top surface ofthe substrate and a second portion buried below the top surface of thesubstrate. The portion extends above the top surface of the substrateincluding a first region forming a transistor channel, a second regionforming a PTS and a third portion between the top portion of thesubstrate and the PTS.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a partially fabricated deviceshowing fins formed on a substrate in accordance with the presentprinciples;

FIG. 2 is a cross-sectional view of the device of FIG. 1 showing thefins covered by a donor layer and a dielectric layer formed between thefins in accordance with the present principles;

FIG. 3 is a cross-sectional view of the device of FIG. 2 showing thefins mixed with Ge in accordance with the present principles;

FIG. 4 is a cross-sectional view of the device of FIG. 3 showing spacersformed on sidewalls of the fins down to the dielectric layer inaccordance with the present principles;

FIG. 5 is a cross-sectional view of the device of FIG. 4 showing thedielectric layer recessed to form gaps below the spacers in accordancewith the present principles;

FIG. 6 is a cross-sectional view of the device of FIG. 5 showing punchthrough stopper doping in the gaps below the spacers in accordance withthe present principles;

FIG. 7 is a cross-sectional view of the device of FIG. 6 showing punchthrough stoppers formed in accordance with the present principles;

FIG. 8 is a cross-sectional view of the device of FIG. 7 showing thegaps filled with a dielectric material in accordance with the presentprinciples;

FIG. 9 is a cross-sectional view of the device of FIG. 8 showing thespacers and a hard mask removed in accordance with one embodiment;

FIG. 10 is a cross-sectional view of the device of FIG. 8 showing thespacers recessed into the dielectric layer in accordance with anotherembodiment;

FIG. 11 is a cross-sectional view showing a completed device inaccordance with the present principles; and

FIG. 12 is a block/flow diagram showing a method for forming a punchthrough stopper in accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, devices and methods areprovided to localize punch through stopper (PTS) doping. In accordancewith particularly useful embodiments, localizing PTS doping assists inminimizing dopant diffusion into a channel region of a fin field effecttransistor (finFET). One embodiment employs SiGe fins with spacers. Thespacers are employed to protect sidewalls of the SiGe fins during adoping process. A dielectric layer is recessed to expose lower portionsof the fins to n-type dopants during a doping process. The n-typedopants are employed as a PTS region, which acts as a barrier for dopantdiffusion between adjacent portions of the fin.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements may be included in the compound and stillfunction in accordance with the present principles. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a cross-sectional view of apartially fabricated semiconductor device 10 is illustratively shown.The device 10 includes fins 14 formed on or etched from a bulk substrate12. In the embodiment shown, the fins 14 have been formed using apatterned hard mask 16 (e.g., SiN or other material). The substrate 12includes a monocrystalline Si substrate. It should be understood thatother embodiments may include the formation of fins on other materialsor substrate structures.

The hard mask 16 may be patterned using lithographical processingtechniques. A reactive ion etch process may be employed to etch thesubstrate 12 to form fins 14. In other embodiments, epitaxial growth maybe employed to increase the substrate 12 followed by hard mask formationand etching to form fins 14.

Referring to FIG. 2, a dielectric layer 20 is formed between the fins 14on the substrate 12. In one embodiment, the dielectric layer 20 isgrown. In another embodiment, the dielectric layer is deposited,planarized and recessed or etched to the base of the fins 14. Thedielectric layer 20 may include an oxide, preferably a silicon oxide. Adonor layer 18 of, e.g., SiGe is conformally formed over the hard mask16 and fins 14. The donor layer 18 is then subjected to a condensationanneal which heats the layer to between about 400 to about 1050 degreesC. for multiple cycles. Each cycle is less than about a minute. Thecondensation process is performed in the presence of oxygen, such thatthe Si of the layer 18 reacts with oxygen and is removed while the Ge inthe SiGe layer 18 diffuses into the Si material of the fins 14 and thesubstrate 12.

Referring to FIG. 3, SiGe fins 22 remain after removal of the SiGe layer18 by oxidation. The fins 22 extend down into the substrate 12. The fins22 may include a high concentration of Ge. The amount of Ge in the fins22 may be controlled by the anneal temperature, the thickness of thelayer 18, the amount of time for each cycle and the number of cycles,among other things. The Ge concentration may be between about 10 at % toabout 60 at % Ge.

Referring to FIG. 4, a spacer dielectric layer 24 is conformallydeposited over the fins 22 and removed from the hard mask 16 and thedielectric layer 20 by an etch process. The etch process may include areactive ion etch (RIE) or other etch process that removes the spacerdielectric layer 24 from horizontal surfaces leaving the spacerdielectric layer 24 on vertical surfaces of the fins 22. The spacerdielectric layer 24 may include SiN although other materials may beemployed. The materials employed for the dielectric layer 20 need to becapable of selective removal relative to the spacer dielectric layer 24.

Referring to FIG. 5, the dielectric layer 20 is etched selectively tothe spacers 24 and the fins 22. The dielectric layer 20 may be wet ordry etched to uniformly recess the layer 20 to expose the fins 22 belowthe spacers 24. The dielectric layer 20 may include a thickness ofbetween about 50 nm to about 200 nm. The etching of the dielectric layer20 may remove enough material to form a gap 26 between the dielectriclayer 20 and the spacers 24. The gap 26 should be sized to provide anarea where a 10 nm to 50 nm punch through stopper (PTS) will be formed,as will be described. The formation of the gap 26 is a self-alignedprocess, wherein the gap 26 opens between the spacer 24 and thedielectric layer 20 upon the removal of the dielectric layer 20.

Referring to FIG. 6, the device 10 is exposed to a plasma dopingprocess. Dopants 28 in the plasma interact with all surfaces and arepresent in the gaps 26. The dopants 28 diffuse into the fins 22 in theexposed areas of the gaps 26 to form PTS regions 30. The PTS regions 30are self-aligned on the fin 22 as a result of the self-alignment of thegaps 26. The type of dopants 28 depend on the type of device beingfabricated. In one particularly useful embodiment, the devices beingfabricated include p-type FETs (PFETs). For PFETs, the dopants mayinclude As, P, or other n-type dopants. The dopant concentration for thedopants 28 should be between about 10¹⁸ to about 10²⁰/cm³. The dopantconcentration may be adjusted as needed. It should be understood thatthe dopants 28 may include p-type dopants for PTS regions 30 for n-typeFETs (NFETs).

Referring to FIG. 7, the dopants 28 diffused into the fins 22 areactivated. Activation of the dopants in the PTS regions 30 may beperformed by performing an anneal process. In one embodiment, thedopants 28 are activated by performing a laser anneal process. Theactivation may be performed by raising the temperature to, e.g., about500-1000 degrees C. for 30 seconds, although other anneal times andtemperatures may be employed. The activation process makes the PTSregions 30 more homogenous and assists is reducing mass transfer acrossthe PTS regions 30. In other words, PTS regions 30 prevent punch throughof dopants between regions 32 and 34 of the fin 22.

Referring to FIG. 8, a dielectric fill 36 is deposited between the fins22 to fill gaps 26. The dielectric fill 36 is planarized and recessedback to a desired height. The dielectric layer 36 will protect the PTSregions 30 when the spacers 24 are removed as will be described in thenext step.

Referring to FIG. 9, in one embodiment, the spacers 24 and the hard mask16 are removed leaving the top and sidewalls of each fin 22 exposed. Theregion 32 of the fin 22 forms a channel for a finFET device. Inaccordance with the present principles, a high Ge concentration SiGechannel 32 is formed that enhances performance especially in PFETs byproviding high charge mobility. PTS doping with n-type dopants is wellcontrolled by employing a spacer mask (spacers 24, FIG. 7) and therecessed dielectric layer (20, FIG. 7) to provide n-type dopants at alocalized position.

Referring to FIG. 10, in another embodiment, instead of completelyremoving the spacers 24 and the hard mask 16 as in FIG. 9, the spacers24 may be recessed down into the dielectric layer 20. The hard mask 16may then be employed for further processing including removing thedielectric layer 20.

Processing can continue in accordance with known processing techniquesto form source and drain regions, gate structures, contacts, silicidescontact liners and metallizations. The devices formed using the channels32 may include PFETs or NFETs for fin structures. The fin structurespreferably include SiGe channels; however, other materials may beemployed using similar processing steps.

It should be understood that the present principles may be applied toany process where spacers can be employed as vertical etch or dopantmasks.

Referring to FIG. 11, a cross-sectional view taken along a fin 22 isshown in accordance with one illustrative embodiment. A gate structure46 includes a gate dielectric 40, spacers 42 and a gate conductor 44formed over the fin 22 (into and out of the page). Source and drainregions 48 are disposed adjacent to the fin 22. The gate dielectric 40may include a high-k dielectric material. The spacers 42 may include anitride material. A cap layer 50 may also be employed and may include anitride, similar to the spacers 42. A region 52 includes SiGe formed inthe substrate 12 by the Ge condensation process. Additional processingincludes forming interlevel dielectric materials, contacts,metallizations, etc., as needed.

Referring to FIG. 12, a method for doping PTSs is shown in accordancewith the present principles. In some alternative implementations, thefunctions noted in the blocks may occur out of the order noted in thefigures. For example, two blocks shown in succession may, in fact, beexecuted substantially concurrently, or the blocks may sometimes beexecuted in the reverse order, depending upon the functionalityinvolved. It will also be noted that each block of the block diagramsand/or flowchart illustration, and combinations of blocks in the blockdiagrams and/or flowchart illustration, can be implemented by specialpurpose hardware-based systems that perform the specified functions oracts or carry out combinations of special purpose hardware and computerinstructions.

In block 102, fins are formed in a monocrystalline substrate. Thesubstrate preferably includes Si. The fins are preferably formed fromthe substrate and are an integral part of the substrate.

In block 104, a dielectric layer is formed at a base portion between thefins. The dielectric layer may include silicon dioxide.

In block 106, in one embodiment, Ge may be mixed in the fins to formSiGe fins, wherein the SiGe fins extend into the monocrystalline Sisubstrate below the dielectric layer. In block 108, the mixing mayinclude thermal mixing or condensation of Ge to form the SiGe fins. Thismay include depositing a SiGe layer on the fins and oxidizing andannealing the SiGe layer to condense Ge and diffuse the Ge into thefins. The SiGe fins extend into the monocrystalline substrate below thedielectric layer.

In block 110, spacers are formed on sidewalls of the fins down to a topportion of the dielectric layer. In block 112, the dielectric layer isrecessed (thinned) to form gaps between the top portion of thedielectric layer and the spacer to expose the fins in the gaps. In block114, the fins are doped through the gaps to form PTSs in the fins. Thedoping may include plasma doping the sides of the fins through the gaps.

In block 116, the PTSs are activated, e.g., by laser annealing. In block118, the gaps are filled with a same dielectric material as thedielectric layer, which is etched back to a position on the spacers. Inblock 120, the spacers are removed from sidewalls of the fins to exposea channel portion of the fin above the PTSs. In block 122, the spacersare recessed down to the dielectric layer to expose a channel portion ofthe fin above the PTSs.

In block 124, a gate structure is formed over a channel portion of thefin. The gate structure may include a gate conductor with spacers alongsidewalls and a gate dielectric in contact with the fin. The spacers mayinclude a silicon nitride material. In block 126, source and drainregions are formed adjacent to the gate and channel portions. In block128, processing continues as needed.

Having described preferred embodiments for localized and self-alignedpunch through stopper doping for finFETs (which are intended to beillustrative and not limiting), it is noted that modifications andvariations can be made by persons skilled in the art in light of theabove teachings. It is therefore to be understood that changes may bemade in the particular embodiments disclosed which are within the scopeof the invention as outlined by the appended claims. Having thusdescribed aspects of the invention, with the details and particularityrequired by the patent laws, what is claimed and desired protected byLetters Patent is set forth in the appended claims.

1. A method for doping punch through stoppers (PTSs), comprising:recessing a dielectric layer to form gaps between a top portion of thedielectric layer and a spacer formed on sidewalls of fins to expose thefins in the gaps; and doping the fins through the gaps to form PTSs inthe fins, wherein a doped region extends from a bottom surface of thePTSs into a substrate.
 2. The method as recited in claim 1, wherein thesubstrate includes monocrystalline Si, and the method further comprisesmixing Ge in the fins to form SiGe fins.
 3. The method as recited inclaim 2, wherein mixing Ge in the fins to form SiGe fins includesdepositing a SiGe layer on the fins and oxidizing the SiGe layer tocondense Ge and diffuse the Ge into the fins.
 4. The method as recitedin claim 2, wherein the SiGe fins extend into the monocrystallinesubstrate below the dielectric layer.
 5. The method as recited in claim1, wherein doping the fins includes plasma doping sides of the finsthrough the gaps.
 6. The method as recited in claim 1, furthercomprising annealing the PTSs to activate the PTSs.
 7. The method asrecited in claim 1, further comprising filling the gaps with a samedielectric material as the dielectric layer and etching back thedielectric layer to a position on the spacers.
 8. The method as recitedin claim 7, further comprising removing the spacers to expose a channelportion of the fin above the PTSs.
 9. The method as recited in claim 7,further comprising recessing the spacers down to the dielectric layer toexpose a channel portion of the fin above the PTSs.
 10. A method fordoping punch through stoppers (PTSs), comprising: forming fins extendinginto a substrate below a dielectric layer; forming spacers on sidewallsof the fins down to a top portion of the dielectric layer; recessing thedielectric layer to form self-aligned gaps between the top portion ofthe dielectric layer and the spacer to expose the fins in the gaps;plasma doping the fins through the gaps to form PTSs in the fins,wherein a doped region extends from a bottom surface of the PTSs intothe substrate; activating the PTSs by annealing.
 11. The method asrecited in claim 10, wherein forming the fins includes mixing Ge in thefins to form SiGe fins by depositing a SiGe layer on the fins andoxidizing the SiGe layer to condense Ge and diffuse the Ge into thefins.
 12. The method as recited in claim 10, further comprising removingthe spacers to expose a channel portion of the fin above the PTSs. 13.The method as recited in claim 10, further comprising recessing thespacers to expose a channel portion of the fin above the PTSs.
 14. Themethod as recited in claim 10, wherein the fins include a channelportion on a first side of the PTS and a second portion on a second sideof the PTS formed in the substrate.
 15. The method as recited in claim14, wherein the channel portion includes a p-type fin field effecttransistor channel and the plasma doping includes n-type dopants.
 16. Afin device having a punch through stopper (PTS), comprising: a findisposed between source and drain regions and adjacent to a gatestructure; the fin having a first portion extending above a top surfaceof a substrate and a second portion buried below the top surface of thesubstrate; and the first portion extending above the top surface of thesubstrate including a PTS region, wherein the PTS region includes adoped region extending from a bottom surface of the PTS into thesubstrate.
 17. The device as recited in claim 16, wherein the PTS regionis between 10 nm and 50 nm in thickness.
 18. The device as recited inclaim 16, wherein the device includes a p-type fin field effecttransistor.
 19. The device as recited in claim 18, wherein the PTSregion includes n-type dopants having a concentration of between about1×10¹⁷ and 1×10²⁰ cm³.
 20. The device as recited in claim 16, whereinthe fin device includes a gate structure including a gate dielectric andgate conductor formed on top of the fin.